Automatic screw fastening uses a screwdriver having a bit engageable with a recess of a screw to fasten the screw that can be inserted into a screw hole in a work. In order to inexpensively supply works, a supply of a small and low-priced automatic screw fastener having an increased throughput is important.
In order to fasten a plurality of screws, the fastening time of a certain screw may need to shift from the fastening time of another screw. For example, as depicted in FIG. 14, in screwing a disc-shaped lid 1 having a packing (not depicted) on the bottom, screws 2 arranged around the lid 1 are first fastened completely and then a center screw 3 is fastened so as to reduce a deformation and a position shift of the lid 1. For similar reasons, as depicted in FIG. 15, in screwing a rectangular lid 6, an internal screw 8 can be fastened after screws 7 arranged around the lid 6 are fastened. For example, the lid depicted in FIG. 14 may be a clamp ring configured to clamp discs in a hard disc drive (“HDD”). The screw 8 depicted in FIG. 15 may be used to fix a head stack assembly (“HSA”) having a carriage mounted with a head and configured to rotate in the HDD.
Prior art include, for example, Patent Documents 1-7:
Patent Document 1: Japanese Patent No. 3569619.
Patent Document 2: Japanese Patent No. 2959244.
Patent Document 3: Japanese Laid-open Patent Publication No. 11-188655.
Patent Document 4: Japanese Patent No. 2959582.
Patent Document 5: Japanese Laid-open Patent Publication no. 7-308865.
Patent Document 6: PCT International Publication No. 2007/099628 (corresponding to U.S. Pat. No. 7,665,366).
Patent Document 7: PCT International Publication No. 2007-105257 (corresponding to U.S. Pat. No. 7,586,284).
In order to fasten two types of screws at different times in an automatic multi-axial screw fastener, it is conceivable to provide two fastening steps for these screws. In this case, the first step fastens the first screw and the next step fastens the second screw. However, these two fastening steps require two separate elevator units for a first screwdriver (or bit) configured to fasten the first screw and a second screwdriver (or bit) configured to fasten the second screw, causing the screw fastener to be large and expensive. In addition, when the second step follows the first step, a total fastening time period becomes longer and the throughput lowers.